Folic acid-mediated metabolism of leukemic and other tumor cells has been the target of chemotherapy with antifolates. Lont-term effective remissions, however, have been thwarted by drug toxicity and the emergence of drug resistant populations during therapy. In addition to these problems, the treatment of solid tumors with antifolates is rendered essentially noneffectual by the antifolate-refractory nature of most solid tumors. A new approach is the therapy of cancer with enzymes directed toward the depletion of malignant cells of their tetrahydrofolate coenzyme pools. Bacterial enzymes, isolated in initial studies for their folate-transforming activity, will be evaluated as therapeutic agents with the methotrexate-refractory Walker 256 rat carcinosarcoma. The effects of dietary serine- and methionine-deprivation on the activity of the enzymes in vivo will be investigated to determine whether concurrent depletion of the plasma of any one of these amino acids could improve the therapeutic effectiveness of the folate- degrading enzymes. These experiments and studies dealing with nutritional and biochemical differences in folate-dependent metabolism of normal and neoplastic cells will guide the continued search for appropriate enzymes. The project will include also studies with antifolate-transforming enzymes with potential as rescue agents subsequent to high doses of methotrexate, to alleviate the toxicity. Enzymic transformation of antifolates in vitro to less toxic agents with unaltered therapeutic value will be explored.